Specific to the topic, I have to agree with jeanne.

Here's another example. Even more basic. You are in the midst of a scene you've done many times before. Unknown to you, since the last time, your sub has had a traumatic event occur in her life, and she just can get into that state of mind that allows her to submit. Suddenly she is only aware of the pain and/or humiliation and is experiencing none of the pleasure. Whether she safewwords or not, all you as the dominant know is that something is going wrong... yet if you've been kept abreast of events in the sub's life, you can deal with the changing environment.

If not, it can be a lever that tears the two apart. It is a situation that I, as a dominant, feel incumbent to share with my sub when it happens to me. If I'm not being my typical vibrant self, how would s/he know she hadn't done something wrong.

And again... we come around to communications. It's key to so many many facets of a successful relationship of any kind.